Ultrasonic energy

Ultrasound - sound waves >20kHz

• Can be used to transmit energy surgically to tissues for cutting and coagulating
• Effect created by mechanical oscillation - alternating current delivered to an ultrasound transducer in the handle of the device. This is used to oscillate some metal cylinders, attached to a solid contiguous shaft ending in a blade or jaw, which oscillates in concert with the cylinders
• For harmonic scalpel, it vibrates about 55kHz

Advantages

• Haemostasis and division of unsupported vascular tissues <=5mm in diameter
• Versatile
• No tissue sticking
• Minimal thermal injury
• No current through patient's body

Disadvantages

• Heat can be retained in shaft
• Blade fatigue or fracture can occur
• Expensive

Settings

• Length of excursion of blade or jaw of device - can typically be adjusted between 50 and 100 micrometres. Longer blade excursion will transmit more mechanical energy to the adjacent tissue and result in greater cutting efficiency.
  • MAX = more rapid cutting and less thermal spread, but minimises haemostasis.
• Less pressure in jaws = less cutting and more tissue coagulation

Mechanisms

• Cavitation
  • Intracellular ions oscillate to the point that the resulting frictional forces cause an elevation of the temperature of the cellular contents to 100 degrees
  • The cell vapourises and ruptures
  • Gaseous products of vaporisation can serve to expand the tissue planes
• Coagulation
• Cutting
  • Mostly occurs due to mechanical interaction between oscillating tip and the tissue